1. Field of the Invention
The present invention relates to a substantially frequency-independent aberration correcting antenna arrangement and, more particularly, to an antenna arrangement which comprises, in sequence along a feed axis thereof, a large offset main reflector, a pair of subreflectors and feeds for communicating with several, spaced-apart remote locations. Large aberrations caused by the main reflector are corrected by disposing one subreflector to form a small image of the main reflector and disposing the second subreflector at the image location and shaped to correct for the aberrations caused by the main reflector.
2. Description of the Prior Art
Except for possibly the axial beam of an antenna, reflectors generally will introduce some sort of aberration if the feedhorn is located away from the geometrical focus. consequently, the wavefront of an off-axis beam is not planar. This is especially true in a multibeam reflector antenna system. Antenna systems, however, have been previously devised to correct for certain aberrations which have been found to exist.
U.S. Pat. No. 3,146,451 issued to R. L. Sternberg on Aug. 25, 1964 relates to a microwave dielectric lens for focusing microwave energy emanating from a plurality of off-axis focal points into respective collimated beams angularly oriented relative to the lens axis. In this regard also see U.S. Pat. No. 3,737,909 issued to H. E. Bartlett et al on June 5, 1973.
Other antenna system arrangements are known which use subreflectors and the positioning of feedhorns to compensate for aberrations normally produced by such antenna systems. In this regard see, for instance U.S. Pat. Nos. 3,688,311 issued to J. Salmon on Aug. 29, 1972; 3,792,480 issued to R. Graham on Feb. 12, 1974; and 3,821,746 issued to M. Mizusawa et al on June 28, 1974.
U.S. Pat. No. 3,828,352 issued to S. Drabowitch et al on Aug. 6, 1974 relates to microwave antennas including a toroidal reflector designed to reduce spherical aberrations. The patented antenna structure comprises a first and a second toroidal reflector centered on a common axis of rotation, each reflector having a surface which is concave toward that common axis and has a vertex located in a common equatorial plane perpendicular thereto.
U.S. Pat. No. 3,922,682 issued to G. Hyde on Nov. 25, 1975 relates to an aberration correcting subreflector for a toroidal reflector antenna. More particularly, an aberration correcting subreflector has a specific shape which depends on the specific geometry of the main toroidal reflector. The actual design is achieved by computing points for the surface of the subreflector such that all rays focus at a single point and that all pathlengths from a reference plane to the point of focus are constant and equal to a desired reference pathlength.
An arrangement was disclosed in the article "A Reflector Antenna Corrected for Spherical, Coma and Chromatic Aberrations" by A. R. Panicali et al in Proceedings of the IEEE, Vol. 59, No. 1, February, 1971, at pp. 311-312 where a corrugated reflector with varying depths of corrugations was suggested.
In the article "Astigmatic Correction by a Deformable Subreflector" by W-Y Wong et al in AP-S International Symposium, Vol. II, Seattle, Wash. 1979, at pp. 706-709, a mechanically deformable subreflector is suggested for providing a first order astigmatic correction. Other astigmatic correction arrangements have been disclosed in, for example, U.S. Pat. Nos. 4,145,695 issued to M. J. Gans on Mar. 20, 1979 and 4,224,626 issued to R. L. Sternberg on Sept. 26, 1980. The Gans patent provides an astigmatic launcher reflector for each off-axis feedhorn which has a reflector having a curvature and orientation of its two orthogonal principal planes of curvature which are chosen in accordance with specific relationships. The Sternberg patent uses a lens having an elliptical periphery and surfaces defined by a system of nonlinear partial differential equations.
U.S. Pat. No. 4,166,276 issued to C. Dragone on Aug. 28, 1979 relates to an offset antenna having improved symmetry in the radiation pattern and comprising a curved focusing main reflector, at least one conic subreflector and a feedhorn; the combination of these elements being oriented such that the feedhorn is disposed at the focal point of the combined confocal reflectors and in a manner to coincide with the equivalent axis of the antenna system. Such arrangement allegedly eliminates astigmatism to a first order approximation.
More recently, U.S. Pat. No. 4,339,757 issued to T. Chu on July 13, 1982 and allowed U.S. patent application Ser. No. 209,944 filed on Nov. 24, 1980 for E. A. Ohm, now U.S. Pat. No. 4,343,004, each disclose different astigmatic correction means comprising a first and a second doubly curved subreflector which are curved in orthogonal planes to permit the launching of an astigmatic beam of constant size and shape over a broadband range.
The foregoing aberration correction arrangements, however, are primarily designed to provide such correction generally for certain particular feed locations. The problem remaining in the prior art is to provide an antenna arrangement for multibeam transmission which will correct for aberrations at multiple feeds near each other.